The alkylation of aromatics with olefins to produce monoalkylaromatics is a well developed art which is practiced commercially in large industrial units. One commercial application of this process is the alkylation of benzene with ethylene to produce ethylbenzene which may be subsequently used to produce styrene. Another application is the alkylation of benzene with propylene to form cumene (isopropylbenzene) which is subsequently used in the production of phenol and acetone. Ethyltoluene and xylene are also important alkylation products.
Often the feedstock to such an aromatic conversion process will include an aromatic component or alkylation substrate, such as benzene, and a C2 to C5 olefin alkylating agent. In the alkylation zone, the aromatic feed stream and the olefinic feed stream are reacted over an alkylation catalyst to produce alkylated benzene. Polyalkylated benzenes are separated from monoalkylated benzene product and recycled to a transalkylation zone and contacted with benzene over a transalkylation catalyst to yield monoalkylated benzenes.
The catalysts for such alkylation or transalkylation reactions generally comprise zeolitic molecular sieves. U.S. Pat. No. 4,891,458 discloses the presence of a catalyst comprising zeolite beta. U.S. Pat. No. 5,030,786 discloses an aromatic conversion process employing zeolite Y, zeolite omega and zeolite beta molecular sieve catalyst. U.S. Pat. No. 4,185,040 discloses the alkylation of benzene to produce ethylbenzene or cumene employing zeolites such as molecular sieves of the X, Y, L, B, ZSM-5 and Omega crystal types. U.S. Pat. No. 4,774,377 discloses an aromatic conversion process involving alkylation over a catalyst comprising a solid phosphoric acid component followed by transalkylation using aluminosilicate molecular sieve transalkylation catalysts including X, Y, ultrastable Y, L, Omega, and mordenite zeolites.
Typical commercial alkylaromatic production processes are designed and operated to achieve an overall olefin conversion of 100%. U.S. Pat. No. 5,215,725; U.S. Pat. No. 5,113,031 and U.S. Pat. No. 5,086,193 describe catalytic packed bed distillation columns that provide essentially complete conversion of olefin. U.S. Pat. No. 5,446,223 describes a catalytic distillation column that can achieve about 90% olefin conversion but which must then be followed by a finishing reactor to convert the remaining olefin to product. The primary motivation for complete olefin conversion is efficient use of the olefinic feed which accounts for a substantial portion of the cost of production for these processes. Moreover, the monoalkylbenzene selectivity is directly related to benzene-to-olefin ratio and is limited by chemical equilibrium. U.S. Pat. No. 5,756,873 contends that olefins may oligomerize over the solid acid catalyst to form heavier compounds that may block acid sites and deactivate the catalyst. Consequently, stoichiometric excess of aromatics relative to olefins in the reactor feed has been employed to prevent deactivation and improve selectivity.
Current industrial practice requires measures be taken to prevent olefin breakthrough from the reaction zone. Loading double, triple and even quadruple the volume of catalyst necessary to alkylate all of the olefins prevents olefin breakthrough even as catalyst deactivates while prolonging the interval between fresh catalyst loadings or regeneration. Moreover, reaction temperatures must be increased as the catalyst deactivates to maintain olefin conversion and prevent olefin breakthrough. Additionally, stoichiometric excess of benzene to olefin requires additional utility cost to fractionate the benzene for recycle.
An object of the invention is to provide an alkylation process that maximizes monoalkylate production while minimizing polyalkylate production.
A further object of the invention is to provide an alkylation process that requires smaller catalyst volume and utility cost.